The answer to your question would depend on the humidity level in your home and the plant you are growing. There really is no rule of thumb for misting. If your home is very dry, misting three times a day might be in order, if it is slightly more humid or the plants are kept in a bathroom where the shower is used often then you might not need to mist at all. In my experience misting makes such a mess of the surrounding area that I prefer to use a pebble tray, a humidifer and/or group plants together to try to keep the humidity up for those plants that really need it.